Field of the Invention
The present invention relates generally to implantable drug delivery devices. More particularly, the invention relates to overfill protection systems for preventing the overfilling and over pressurization of a reservoir of an implantable drug delivery device.
Description of Related Art
Implantable drug delivery systems are in widespread use to provide site-specific and/or sustained delivery of beneficial agents to address adverse patient conditions. Such delivery systems may include implantable infusion pumps, which typically include a pressurized drug reservoir and some form of fluid flow control. One example of an implantable infusion pump is the SYNCHROMED.TM. pump manufactured by Medtronic, Inc. of Minneapolis, Minn.
Typically, implantable drug delivery devices are periodically refilled in situ and percutaneously using a refill device, such as a hypodermic syringe inserted into a refill chamber of the device. A common problem related to refilling is the potential for overfilling or over pressurization of the device. Typically, the clinician who refills the device must rely on tactile pressure to sense that the drug reservoir has been filled to capacity. Any additional fluid injected into the device beyond capacity of the reservoir may cause severe damage to the drug administration device and may cause other adverse consequences.
Prior art overfill prevention devices, such as those described in U.S. Pat. No. 5,158,547, the entire writing of which is incorporated herein by reference, are exemplified by FIG. 1, which is a cross-section of a drug delivery device in the form of an implantable infusion pump 10. Generally, a valve 12 having a rigid valve stem 14 is fastened, usually by welding, to an inner surface 16 of the pump diaphragm or bellows 18. As new drug supply is injected through the septum 20 using a refill device (not shown), the reservoir chamber 22 fills with drug and the surface 16 moves towards a full position (downward in FIG. 1). As the reservoir chamber 22 reaches capacity, the valve 12 is pulled into sealing engagement with a valve seat 26, resulting in a detectable increase in pressure in the refill device and preventing overfilling and/or over pressurization of the drug delivery device 10. Notably, in prior art devices, as the reservoir collapses, the valve 12 and valve stem 14 must travel within the pump housing 30 a distance approximately equal to the distance traveled by the surface 16 of the reservoir 18.
Large reservoir sizes are desirable because they permit more quantity of a given therapeutic agent to be stored in the device and reduce the amount of refills necessary for a given period of time. On the other hand, it is desirable to reduce or maintain the size of the "footprint" of the overall drug delivery device so as to increase the ease of implantation and reduce associated trauma to the living body. As the size of the power and flow control features of drug delivery devices becomes reduced, due to advances in the related arts, there is potential for larger size reservoirs without associated increases in the size of the overall delivery device. However, prior art overfill protection systems are characterized by valve displacement that is equal to the displacement of the reservoir surface and therefore require that the pump housing depth be increased to accommodate the increased valve displacement that would occur with reservoirs of increased depth. Thus, prior art devices present an obstacle to further reduction in delivery device size and increase in reservoir size.
What is needed is an overfill protection system for implantable drug delivery devices which addresses the aforementioned problems. Specifically, what is needed is an overfill protection system for an implantable drug delivery device that does not require an increase in housing depth when the reservoir depth is increased.